1. Field of the Invention
The present invention relates to a motor control apparatus for synchronously controlling a master axis motor for driving a master axis and a slave axis motor for driving a slave axis.
2. Description of the Related Art
In a machine tool, motors are provided for each drive axis of a machine tool, and are driven and controlled by a motor control apparatus. The motor control apparatus controls the motor speed, torque, or rotor position of each of the motors that drive the respective drive axes of the machine tool.
FIG. 7 is a diagram for explaining a tool axis and a work axis in a gear driven machine. The drive axes of the gear turning machine include an axis (tool axis) for driving a tool such as a grinding wheel or a cutter and an axis (work axis) for driving a workpiece. The tool axis and the work axis are mechanically coupled together via a work gear.
It is known to provide a master-slave synchronizing method, such as disclosed in Japanese Unexamined Patent Publication No. H04-42307 for a gear driven machine, in which the motor for driving the tool axis is designated as the master axis motor and the position data detected on the master axis motor is also utilized as a command to the slave axis motor for driving the work axis, thereby controlling the master axis and the slave axis in a phase (rotational angle) synchronized fashion.
FIG. 8 is a basic functional block diagram schematically illustrating a circuit for synchronized control of the master axis and the slave axis according to the prior art. It is to be understood that, throughout the different drawings given herein, the same reference numerals designate component elements having the same functions. In the motor control apparatus 100 according to the prior art, position data and a reference signal with a predetermined fixed period (for example, a one-rotation signal), as analog signals output from a master axis position detector 111 provided for the master axis, are each branched by a branch circuit 110 and supplied to receiving circuits 112 and 152 provided for the master axis and the slave axis, respectively, and computation circuits 113 and 153 connected to the respective receiving circuits 112 and 152 each create angle information representing the “distance traveled from the reference signal position,” and transmit the angle information to a master axis motor control unit 115, which controls the operation of the master axis motor for driving the master axis, and a slave axis motor control unit 155, which controls the operation of the slave axis motor for driving the slave axis, respectively. Then, based on the separately received angle information, the master axis motor control unit 115 and the slave axis motor control unit 155 synchronously control the master axis motor and the slave axis motor so that the phase (rotational speed) is matched between the master axis and the slave axis.
FIG. 9 is a block diagram showing in further detail the configuration depicted in the basic functional block diagram of FIG. 8. A move command value creating unit 122 creates move commands for the master axis motor 114 and the slave axis motor 154, respectively, in accordance with a program stored in a storage unit 121, and supplies the move commands to the master axis motor control unit 115 and the slave axis motor control unit 155, respectively. A position/speed control unit 116 in the master axis motor control unit 115 controls the rotor position (master axis position) and rotational speed of the master axis motor 114 on the basis of the move command, the rotational speed fed back from the master axis motor 114 (master axis speed feedback), and the angle information fed back from the master axis position detector 111 (master axis position feedback). On the other hand, a position/speed control unit 156 in the slave axis motor control unit 155 controls the rotor position (slave axis position) and rotational speed of the slave axis motor 154 on the basis of the move command, the rotational speed fed back from the slave axis motor 154 (slave axis speed feedback), and the angle information supplied from the master axis position detector 111 via the branch circuit 110. As described with reference to FIG. 8, the position data and the reference signal with a predetermined fixed period (for example, a one-rotation signal), output from the master axis position detector 111, are each branched by the branch circuit 110 and supplied to the receiving circuits 112 and 152, and the computation circuits 113 and 153 connected to the respective receiving circuits 112 and 152 create the respective angle information separately.
Another method for controlling by phase-synchronizing the master axis and the slave axis is disclosed, for example, in Japanese Unexamined Patent Publication No. 2005-322076, which synchronizes control while maintaining a desired phase relationship and a desired ratio between different drive axis systems.
There is also proposed a method of synchronized control capable of achieving quick phase synchronization between the master axis and the slave axis, as disclosed in Japanese Unexamined Patent Publication No. H08-202420.
As described earlier, in the master-slave synchronizing method, the position data and the reference signal with a predetermined fixed period, as analog signals output from the master axis position detector 111 provided for the master axis, are each branched and supplied to the receiving circuits 112 and 152 separately provided for the master axis and the slave axis, respectively. It is therefore difficult to synchronize the reference signal receive timing between the receiving circuits 112 and 152 because of variations among parts used in the receiving circuits 112 and 152, different transmission paths used, temperature variations, etc. As a result, the following problem occurs.
FIGS. 10a, 10b, 10c, and 10d are diagrams for explaining how the angle information is generated using the position data output from the master axis position detector. The master axis position detector 111 is constructed, for example, from a rotary encoder, and outputs position data including two signals, i.e., an A-phase signal and a B-phase signal, for example, as shown in FIG. 10a, so as to enable the rotational speed (position) and rotational direction of the master axis to be detected. The master axis position detector 111 outputs a reference signal once in a predetermined fixed period, such as shown in FIG. 10b; the reference signal is, for example, a one-rotation signal. The computation circuits 113 and 153 each read the count value of a position counter, such as shown in FIG. 10c, when the reference signal is received, and again read the count value of the position counter the next time the reference signal is received; then, by calculating the amount of increase from the previous count value, the distance traveled from the reference signal position, such as shown in FIG. 10d, i.e., the angle information within one rotation of the rotor, is computed. However, if, for any one of the above reasons, a displacement occurs in the reference signal receive timing (as indicated by dotted lines in FIG. 10b) between the receiving circuits 112 and 152 separately provided for the master axis and the slave axis, respectively, the count value of the position counter read at the time of the reception of the reference signal becomes different between the master axis and the slave axis, and as a result, a phase displacement occurs in the angle information computed by the respective computation circuits 113 and 153, as shown in FIG. 10d. In this way, since the receiving circuit 112 for the master axis and the receiving circuit 152 for the slave axis are separately provided, if the position data and reference signal output from the same master axis position detector 111 are used, a phase displacement occurs in the angle information computed by the respective computation circuits 113 and 153 because the receive timing is displaced between the receiving circuits 112 and 152. If angle information containing such a phase displacement is used, it is not possible to control the master axis motor 114 and the slave axis motor 154 in a synchronized fashion.
Furthermore, since the prior art master-slave synchronizing method requires the provision of a branch circuit, the cost and size of the motor control apparatus correspondingly increase.